Discarded cigarette butt-derived cobalt-based bifunctional catalysts for simultaneous catalytic oxidation of MgSO₃ and adsorption of Pb(II) in magnesium-based flue gas desulfurization systems

2.1. Materials and chemicals

Discarded cigarette butts, which were used to create activated carbon, were collected from multiple public places (such as cafeterias and playgrounds), to minimize the experimental error, discarded cigarette butts of the same brand were screened as raw material. All reagents used in this study are of analytical grade and can be used directly without further purification. Further details have been provided in Appendix 1 (Supplementary materials).

2.2. Synthesis of discarded cigarette butt-derived activated carbon

Discarded cigarette butt-derived AC was prepared following the method previously reported [26] with some modifications. The discarded cigarette butts were first removed from the wrappers and soaked in absolute ethyl alcohol for 24 h to eliminate some organic substances (e.g., nicotine and tar) and heavy metal ions. To explore the role of KOH as a chemical activation agent in the preparation of AC, three types of activated carbons, named AC-0, AC-1, and AC-2, using different mass ratios of cigarette butt carbon to KOH (1:0/1:1/1:2). A brief preparation process has been shown in Figure 1(a), details of the preparation process can be found in Appendix 2 (Supplementary materials).

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Figure 1.

(a) Schematic diagram of AC preparation, (b) fabrication of catalysts, (c) diagram of MgSO₃ oxidation.

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2.3. Catalyst preparation

The synthetic procedure for the Co/AC catalyst has been depicted in Figure 1(b). The cobalt-based catalyst prepared by the hydrothermal method is defined as Co/AC-Hy. To demonstrate the effects of different preparation methods on the catalytic performance, a cobalt-based catalyst was synthesized using the conventional impregnation method and designated as Co/AC-Im. The catalyst synthesis procedure has been described in Appendix 3 (Supplementary materials).

2.4. Characterizations

A comprehensive set of techniques was employed to characterize and analyze the AC and catalysts. These techniques include X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), N₂ adsorption/desorption, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), Raman spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), and hydrogen temperature-programmed reduction (H₂-TPR). Details have been presented in Appendix 4 (Supplementary materials).

2.5. Catalyst performance test system

The test system for the oxidation of MgSO₃ and the adsorption of Pb²⁺ has been illustrated in Figure 1(c). Detailed information regarding the performance tests can be found in Appendix 5 (Supplementary materials). All experiments were conducted at a longitude of 102°51′16″E, latitude of 24°50′33″N, with a room temperature of 25°C. The turnover frequency (TOF) is defined as mol of oxidized MgSO₃ per mole of Co atom per s). The Pb²⁺ removal efficiency (RE_Pb and the Pb²⁺ adsorption capacity (q_t) were obtained by Eqs. (1-3) respectively.

Where ${\text{n}}_{{\text{oxidized MgSO}}_3}$represents the moles of oxidized MgSO₃ (mol); ${\text{n}}_{\text{Co}}$ is the number of Co active sites (mol), and t is the reaction time (s); ${\text{c}}_0$ is the initial concentration of Pb²⁺ (mg·L^-1); ${\text{c}}_{\text{t}}$ is the concentration of Pb²⁺ at time t (mg·L^-1); V is the volume of the solution (mL); m is the mass of the adsorbent (g).

3.1. Characterization of materials

The investigation began with an analysis of how the activation agent KOH influences pore expansion during AC production. Three types of activated carbons, AC-0, AC-1, and AC-2, were prepared according to different mass ratios of cigarette butt carbon to KOH. Their nitrogen adsorption and desorption and pore size distributions have been shown in Figure S1 Each of the three samples displays type IV isotherms, a typical mesoporous material characteristic. Moreover, the pore size distribution shows that it is a micro-mesoporous material. The Pore texture parameters of the samples determined for Nsorption are detailed in Table S1 Notably, compared to AC-0 (495.90 m²·g^-1), AC-2 has a higher specific surface area (916.97 m²·g^-1). Additionally, AC-2 has a larger microporous volume (V_micro = 0.37 cm³·g^-1) and pore size (2.43 nm), with a rich pore structure. The results indicate that the prepared AC possesses a highly porous structure and a large specific surface area. KOH is essential in developing this abundant porous structure. KOH acts as an effective oxidizing agent, facilitating the conversion of carbon into carbonates ($6\text{KOH}+2\text{C}\leftrightarrow 2{\text{K}}_2{\text{CO}}_3+2\text{K}+3{\text{H}}_2$), which create pores in the carbon framework through an etching process [29]. The large specific surface area and unique micro/mesoporous structures enhance the exposure of active sites and offer numerous adsorption sites. This reduces the resistance in the mass transfer process and promotes the swift transfer of ions, resulting in improved catalytic performance [30].

The scanning electron microscopy (SEM) images (Figures 2a and b) were employed to reveal the morphology and structural features of AC-2. The AC-2 samples exhibited smooth surfaces featuring interconnected and uniform lamellar structures. This distinctive layered configuration significantly reduces the mass transfer between reactants [8]. Notably, the surface of these flakes clearly shows the presence of pore structures, indicating that such pore structures, formed during the high-temperature pyrolysis process. Energy dispersive spectrometer (EDS) mapping images (Figure 2c) and elemental analysis (Figure 2d) revealed that the AC-2 surface is primarily composed of C and O elements, which are uniformly distributed. The SEM images of Co/AC-Im and Co/AC-Hy were analyzed separately. The surface of Co/AC-Im exhibits a rough texture with numerous pores and displays a lamellar structure. Conversely, Co/AC-Hy exhibits a smoother texture, although it also shares a similar lamellar structure. This unique lamellar structure significantly reduces the resistance encountered by reactants and products during their diffusion to active sites [31], thus allowing for enhanced oxidizing performance. Furthermore, the agglomeration of metal particle clusters was observed on the surface of Co/AC-Im (Figures 2e and f). This phenomenon may be attributed to the weak interaction between cobalt and the support in the catalyst prepared by the impregnation method, which results in the partial agglomeration of cobalt oxide. In contrast, no noticeable agglomeration of metal particles was observed in the Co/AC-Hy sample (Figures 2g and h). Indicating that cobalt oxide is uniformly distributed on the surface of support, demonstrating excellent dispersion. Additionally, the EDS mapping images (Figures 2i and j) further corroborated this observation, revealing the uniform distributions of elements C, O, and Co. The distribution of element Co in Co/AC-Im was primarily centralized, covering almost the entire carbon surface and exhibiting significant aggregation. In contrast, the Co distribution in Co/AC-Hy was more uniform across the surface. This observation is further supported by the findings from the elemental analysis (Figures 2k and l). A substantial amount of Co was found on the surface of Co/AC-Im, attributable to the clustering of cobalt oxides on the catalyst surface. Conversely, only trace quantities of Co were detected on the surface of Co/AC-Hy, confirming a uniform distribution of Co species across the catalyst surface. Transmission electron microscopy (TEM) images (Figures 2m and n) reveal the typical lamellar structures of Co/AC-Im and Co/AC-Hy. The Co/AC-Im structure exhibits metallic agglomerates, which is consistent with the SEM results. For Co/AC-Hy, due to their thickness, they are almost transparent to the electron beam [8] Additionally, high-resolution transmission electron microscopy (HRTEM) images (Figures 2o and p) reveal a lattice stripe spacing of 0.24 nm, which corresponds to the lattice of well-crystallized CoO (111) [32], speculating the formation of crystalline CoO on the materials’ surface. Subsequent XRD analysis further confirmed the presence of the CoO substance (Figure 3a).

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Figure. 2.

SEM images of (a,b) AC, (c,d) Co/AC-Im, (e,f) Co/AC-Hy. EDS mapping of (g) AC, (h) Co/AC-Im, (i) Co/AC-Hy (showing C, O, Co distribution). EDS elemental analysis of (j) AC, (k) Co/AC-Im, (l) Co/AC-Hy. TEM images of (m,n) Co/AC-Im, (o,p) Co/AC-Hy.

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Figure 3

(a) XRD pattern, (b) FT-IR spectra, (c) Raman spectrum, (d) XPS survey analyses, (e) Co 2p XPS spectra, (f) O 1s XPS spectra, (g) H₂-TPR profiles, (h) N₂ adsorption-desorption isotherms, (i) pore size distribution.

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Figure 3(a) presents the XRD patterns for pure AC, Co/AC-Im, and Co/AC-Hy. The broad diffraction peaks at 26.1° correspond to the C (002) planes in the discarded cigarette-derived AC, Co/AC-Im, and Co/AC-Hy. This indicates that discarded cigarette butts form typical amorphous carbon during pyrolysis and maintain this structure throughout the catalyst preparation process [33]. The presence of cobalt species as CoO in both Co/AC-Im and Co/AC-Hy was confirmed by strong diffraction peaks observed at 36.5°, 42.4°, and 61.8°, respectively [34]. This finding is supported by the results of the HRTEM analysis, suggesting that active metal oxides have been successfully bound to the AC support. The dispersibility of the active component is significantly influenced by the functional groups present on the surface of the support, particularly hydrophilic groups like hydroxyl groups. Figure 3(b) presents the FT-IR of pure AC, Co/AC-Im, and Co/AC-Hy. The observed vibrational peaks are 3476, 2958, 2365, 1728, 1575, 1011, and 573 cm^-1. The weak broad peak at 3476 cm^-1 found in all materials is attributed to the O-H stretching of the hydroxyl groups. The peak at 2958 cm^-1 corresponds to the C-H stretching vibration of the AC surface [4]. Additionally, the peaks at 1728 cm^-1 and 1575 cm^-1 are associated with the C=O stretching vibrations of either carboxyl or lactone groups. When cobalt is loaded, there is a noticeable red shift in the absorption peaks, likely due to strong bonding with cobalt species [4,35]. The peak observed at 1011 cm^-1 is ascribed to C-O stretching, while the broad absorption peaks of Co/AC-Im and Co/AC-Hy at 573 cm^-1 are attributed to the bending vibration of the Co-O bond [36]. These oxygen-containing functional groups, which are hydrophilic, facilitate the dispersion of cobalt species on the support [4]. Furthermore, Figure 3(c) illustrates the Raman spectrum of all the samples. Two prominent peaks are observed at 1580 cm^-1 and 1350 cm^-1, which correspond to the G and D bands of the carbon material, respectively. The G band is associated with the symmetric stretching of the sp² carbon atoms and is indicative of graphitic carbon content, whereas the D band is directly related to defects or disorders within the carbon structure [37]. After Co loading, a slight decrease in the intensity area ratio ID/IG was observed, indicating a reduction in the degree of carbon defects [38]. The observation is consistent with the FT-IR characterization results, which indicate that the addition of Co species did not change the type or structure of substances present on the AC surface. This was further validated by the similar spectral structure exhibited by both Co/AC-Im and Co/AC-Hy, which matched that of the AC.

In the XPS spectrum presented in Figure 3(d), distinct binding energy peaks corresponding to C 1s, O 1s, and Co 2p were identified, respectively, indicating that Co/AC-Im and Co/AC-Hy samples are composed of C, O, and Co elements. The Co 2p spectrum (Figure 3e) reveals a spin-orbital splitting of approximately 16 eV between the Co 2p_3/2 peak (780.8 eV) and the Co 2p_1/2 peak (796.7 eV). Additionally, the presence of strong satellite peaks at 786.6 eV and 803.4 eV indicates the existence of Co²⁺ species in both Co/AC-Im and Co/AC-Hy samples [12,39]. The presence of CoO was confirmed, which is consistent with the results obtained from XRD and HRTEM analyses. The Co 2p_3/2 peaks observed at 781.7 eV and 780 eV were identified as corresponding to Co²⁺ and Co³⁺, respectively [12]. The analysis reveals that the content of Co³⁺ in Co/AC-Hy is significantly lower than that in Co/AC-Im, indicating that the hydrothermal process effectively regulates the valence distribution of Co species and inhibits the formation of Co³⁺ species. Related studies have shown that Co²⁺ is the main active component in the multiphase catalytic oxidation reaction of MgSO₃ [40]. The high catalytic activity of Co²⁺ can be attributed to lattice defects. When Co³⁺ in the highly oxidized state is reduced to Co²⁺ by carbon in Co/AC, cobalt ion defects are created in the framework of the lattice structure to maintain charge balance [18,41]. As a result, the increased conversion of Co³⁺ to Co²⁺ generates a greater number of cobalt ion defect sites, which enhances the overall catalytic activity. In addition, it has been shown that appropriately increasing the dispersion of cobalt oxide (CoO_x) facilitates the generation of lattice defects [42]. In Co/AC-Hy, cobalt particles were uniformly distributed along the inner pore surfaces of AC. This distribution allowed for sufficient interaction between carbon and Co³⁺, which facilitated the reduction process and increased the number of active Co²⁺ centers as well as lattice defect sites, thereby enhancing catalytic performance. In contrast, Co/AC-Im displayed a tendency for cobalt particles to agglomerate, leading to blocked pores. This obstruction hindered full contact between carbon and Co³⁺, preventing effective reduction and the formation of lattice defects [43]. Therefore, Co/AC-Hy has theoretically superior performance. Furthermore, active oxygen species (ROS) play a crucial role in enhancing the efficiency of catalysts for sulfite oxidation. The O 1s spectra of Co/AC-Im and Co/AC-Hy were analyzed. Figure 3(f) reveals that the binding energy at 530.9 ± 0.3 eV and 532.6 ± 0.3 eV are attributed to lattice oxygen (O_β) and adsorbed oxygen (O_α), respectively [12,17]. According to the literature [44], O_α significantly contributes to the formation of lattice defects and oxygen vacancies (O_V) on the catalyst surface. These defects and O_V act as effective active sites for oxygen adsorption during the oxidation reaction process [18], thereby facilitating the conversion of sulfite to sulfate. As a result, Co/AC-Hy, which contains a higher amount of O_α, is expected to perform better theoretically than Co/AC-Im.

The reducing properties of Co/AC-Im and Co/AC-Hy were examined using H₂-TPR. As illustrated in Figure 3(g), three cobalt reduction peaks were identified: reduction peak I (220°C - 250°C), reduction peak Ⅱ (300°C - 350°C), and reduction peak Ⅲ (490°C - 520°C). The reduction peaks Ⅰ and Ⅱ in the low-temperature region are ascribed to the two-step reduction of Co₃O₄, which involves the reduction of Co³⁺ to Co²⁺ and Co²⁺ to Co⁰. The reduction peak Ⅲ in the high-temperature region is associated with the interactions between the active component and the AC support [45]. The peak intensity and peak area indicate that Co/AC-Im may contain large aggregates of cobalt species particles. This suggests a weak interaction between the metal oxides and their supports, along with a low dispersion of cobalt oxides [46]. Additionally, the reduction temperature of peak III in the reduction process of Co/AC-Im is observed to shift to a lower temperature. This shift further supports the notion of weak interaction between cobalt and AC, as well as the clustering phenomenon of metal oxides. The findings align with the SEM results, reinforcing each other. The H₂-TPR profiles reveal that the synthesized catalysts facilitate the reduction of CoO_x, suggesting a potentially greater availability of active Co species for sulfite oxidation. Catalysts with a large specific surface area and an abundance of porous structures can provide additional adsorption sites for the adsorption of Pb²⁺ and the oxidation of sulfite. The structural properties of Co/AC-Im and Co/AC-Hy were investigated through N₂ adsorption-desorption analysis. Notably, both Co/AC-Im and Co/AC-Hy display type‐IV isotherm characteristics (Figure 3h), which are typical of micro/mesoporous materials. The samples exhibited a high adsorption capacity in the low-pressure region (p/p₀ < 0.01), indicating a rich microporous structure. Additionally, the adsorption isotherms observed at relative pressures (p/p₀) ranging from 0.3 to 0.9 suggest the presence of mesoporous structures. The pore size distribution (Figure 3i) indicates that Co/AC-Im and Co/AC-Hy are primarily composed of a large number of micropores and smaller mesopores. Co/AC-Hy has a larger specific surface area (465.49 m²·g^-1), in contrast to Co/AC-Im, which has a lower specific surface area (424.14 m²·g^-1) due to the agglomeration of metal particles. The large specific surface area of Co/AC-Hy is believed to enhance the exposure and mass transfer of active sites. This porous nature promotes oxygen ingress into the material, thus facilitating the oxidation reaction with MgSO₃.

3.2 Catalytic performance of MgSO₃ oxidation and Pb²⁺ adsorption

The effectiveness of the catalysts in adsorbing Pb²⁺ from wastewater and their ability to simultaneously control sulfite oxidation while removing Pb²⁺ were examined. Figure 4(a) presents a simple schematic of the reaction process. Initially, the impact of different preparation methods on the oxidation rate of MgSO₃ was investigated, with the results displayed in Figure 4(b). In preliminary experiments, the effect of varying Co loadings on the MgSO₃ oxidation rate was investigated (Figure S2). Based on the experimental results and consideration of cost-effectiveness, a Co loading of 20% is deemed optimal. Interestingly, this study achieved a higher MgSO₃ oxidation rate with a lower Co loading of 20%, compared to the 30% Co loading reported in related research [4]. Therefore, Co₂₀/AC was chosen for subsequent studies. The oxidation rate of MgSO₃ was significantly influenced by the calcination temperatures of the catalysts. As the calcination temperature increases, the MgSO₃ oxidation rate exhibits a “volcanic type” pattern (Figure 4c). The oxidation rate of the catalyst MgSO₃ (0.085 mmol·L^-1·s^-1) was highest at 400°C. This elevated activity at 400°C can be attributed to the form and dispersibility of the Co species present at this temperature. Figure S3 illustrates the TG and DTG curves for the catalyst in the 400°C - 600°C range, which demonstrate the stable presence of CoO species. The XRD patterns of the products, affected by varying calcination temperatures, are depicted in Figure S4. At lower temperatures, the primary form of the Co species in the catalyst is Co₃O₄, while, when the calcination temperature exceeds 400°C, the Co species in the catalyst transition to CoO. In the MgSO₃ multiphase catalytic oxidation reaction, Co²⁺ demonstrates superior catalytic activity compared to Co³⁺, so the catalyst exhibits higher catalytic activity at 400°C. The N₂ adsorption-desorption isotherms showed that the samples obtained at different calcination temperatures exhibit type IV isotherms (Figure S5). Additionally, the pore size distribution analysis reveals that is a typical porous material with a well-developed pore structure. Table S1 provides a summary of the structural properties, specifically the specific surface area, pore volume, and average pore size, of the catalysts. The calcination temperature significantly affects the specific surface area of the catalysts [47]. An increase in calcination temperature results in a rise in specific surface area, peaking at 465.49 cm²·g^-1 when the calcination temperature reaches 400°C. However, as the temperature continues to increase beyond this point, the specific surface area begins to decline. This decrease may be attributed to the collapse of certain pores within the catalyst due to the higher calcination temperature [48]. Consequently, the effects of different methods for introducing Co species on the MgSO₃ oxidation rate and the adsorption performance of Pb²⁺ were carefully examined under optimal conditions regarding Co loading and calcination temperature.

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Figure 4.

Sulfite oxidation performance and Pb²⁺ removal experiments: (a) reaction schematic, (b) sulfite oxidation rate at different calcination temperatures, (c) sulfite oxidation rate without and with Pb²⁺ (${\text{C}}_{{\text{Pb}}^{2+}}$= 2 mg·L^-1), (d) TOF value of the as-prepared catalysts, (e) removal efficiency of Pb²⁺ under different conditions, (f) relationship between equilibrium concentration and removal rate of Pb²⁺(pH = 6, ${\text{C}}_{{\text{MgSO}}_3}$= 50 g·L^-1, ${\text{C}}_{{\text{Pb}}^{2+}}$= 2 - 150 mg·L^-1), (g) relationship between equilibrium concentration and adsorption capacity of Pb^2+.

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The Co/AC-Hy catalyst exhibited superior catalytic activity compared to the Co/AC-Im catalyst, achieving a MgSO₃ oxidation rate of 0.085 mmol·L^-1·s^-1. This represents a significant increase from the uncatalyzed oxidation rate (0.010 mmol·L^-1·s^-1), which was measured at a longitude of 102°51′16″E, latitude of 24°50′33″N, standard room temperature of 25 ± 0.5°C, and without any added catalyst. Excitingly, this performance is notably competitive with most of the AC-loaded cobalt-based catalysts reported in the literature (Table S2). The calculated turnover frequency (TOF) for the oxidation of MgSO₃ catalyzed by Co/AC-Hy was found to be 0.080 s^-1 (Figure 4d), which further indicates the superior catalytic performance of Co/AC-Hy in this reaction. The oxidation rate of MgSO₃ by Co/AC-Im was measured at 0.056 mmol·L^-1·s^-1, which is a fivefold improvement compared to the uncatalyzed baseline. In comparison, Co/AC-Hy exhibits excellent MgSO₃ oxidation performance, which can be attributed to its abundant adsorbed oxygen species (Figure 3f) and Co²⁺ active component (Figure 3e). Additionally, its excellent dispersion of active components enables efficient catalytic oxidation of MgSO₃. Thus, Co/AC-Hy demonstrates outstanding properties for MgSO₃ oxidation.

Moreover, a slight decrease was observed in both the oxidation rate of MgSO₃ and the TOF of the prepared catalysts when MgSO₃ and Pb²⁺ (2 mg·L^-1) were present together. This indicates that Pb²⁺ has a mild inhibitory effect on the oxidation of MgSO₃, which could be attributed to the adsorption of Pb²⁺ onto the catalyst surface, thus occupying some of the reactive sites. The functional groups present in the catalysts exhibit a strong adsorption capacity to adsorb heavy metal ions, and there are likely many vacant surface oxygen sites available for adsorption [8]. Some of these oxygen vacancies are occupied by adsorbed Pb²⁺, which reduces the MgSO₃ oxidation performance. Fortunately, both Co/AC-Im and Co/AC-Hy showed high oxidation rates of MgSO₃ without significant signs of catalyst poisoning. The experimental results indicate that Co/AC-Hy exhibits excellent performance in both MgSO₃-catalyzed oxidation and the simultaneous control of MgSO₃ oxidation and Pb²⁺ adsorption. This bifunctional catalyst is posited to not only enhance the oxidation rate of MgSO₃ but also effectively adsorb Pb²⁺ from the desulfurization slurry. The removal of Pb²⁺ by Co/AC-Hy is depicted in Figure 4(e). In the absence of MgSO₃, Co/AC-Hy exhibited a remarkable removal efficiency for Pb²⁺ within 20 minutes at an initial concentration of 2 mg·L^-1, the removal efficiency was measured at 84.52%, and this value remained largely consistent. Initially, there are many available oxygen sites for adsorption. However, as the reaction progresses, the rate of adsorption may decrease due to the repulsive forces exerted by the Pb²⁺ between the solid and liquid phases [8]. After adding MgSO₃, the efficiency of Pb²⁺ removal decreased, but the effective removal of Pb²⁺ was still achieved at 82.34%. Some sites were used for the oxidation of MgSO₃, leading to a slight reduction in the adsorption effect. The synergistic adsorption performance of Pb²⁺ on Co/AC-Hy was investigated under optimal conditions with initial Pb²⁺ concentration ranging from 2 to 150 mg·L^-1. Figures 4(f, g) illustrate that the adsorption capacity of the equilibrium Co/AC-Hy catalyst increased with higher Pb²⁺ concentration, while the removal efficiency decreased at elevated Pb²⁺ levels. This may be due to a limited number of adsorption sites being occupied. In summary, Co/AC-Hy demonstrated superior synergistic adsorption performance for Pb²⁺, which enhanced the oxidation rate of MgSO₃ and effectively removed the coexisting Pb²⁺ from the slurry, resulting in high-quality MgSO₄·7H₂O products.

3.3. Mechanism insights

3.3.1. Kinetics and mechanism of MgSO₃ oxidation

The oxidation rate of MgSO₃ is influenced by both reaction temperature and pH. To investigate the effect of temperature on the oxidation rate of MgSO₃, experiments were conducted under both non-catalytic (Figure S6a) and catalytic conditions (Figure 5a) by varying the reaction temperature. Increasing the temperature of the reaction system helps accelerate the oxidation process of MgSO₃, resulting in a higher oxidation rate as the temperature rises. As the reaction temperature increases, more activated molecules participate in the reaction per unit of time, which enhances the reaction rate [49]. The slopes of the straight lines were determined by plotting the logarithm of the reaction rate (lnr) against the reciprocal of the temperature (1/T) (Figures S6b and c). The Arrhenius equation was used to calculate the apparent activation energy (E_a) for the reaction under both non-catalytic and catalytic conditions were (18.80 kJ·mol^-1) and (4.97 kJ·mol^-1), respectively. These results indicate that the Co/AC-Hy multiphase catalyst significantly reduced the apparent activation energy of the MgSO₃ oxidation reaction, demonstrating excellent catalytic performance.

Figure S6

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Figure 5.

a) MgSO₃ oxidation rate at different temperatures, (b) MgSO₃ oxidation rate at different pH, (c) Langmuir equilibrium isotherm adsorption isotherm modeling, (d) Freundlich equilibrium isotherm adsorption isotherm modeling, (e) pseudo-first-order kinetics fitting, (f) pseudo-second-order kinetics fitting and (g) The hypothesized mechanism of sulfite oxidation process.

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The effect of pH on the oxidation reaction of MgSO₃ has been illustrated in Figure 5(b). In a strongly acidic environment (pH < 4), the ${\text{SO}}_3^{2-}$ could be converted to SO₂. There is a risk of SO₂ release from the solution. In a weakly acidic environment (4 < pH < 6), H⁺ reacts with ${\text{SO}}_3^{2-}$ and ${\text{HSO}}_3^{-}$ in the solution, which shows the rate of oxidation of ${\text{SO}}_3^{2-}$ [13]. When the solution shifts to an alkaline (pH > 10), the high OH^- concentration significantly decreases the solubility of MgSO₃, making it challenging for the compound to engage in the catalytic oxidation reaction. Therefore, the optimum pH for the catalytic oxidation reaction of MgSO₃ is approximately 8, which is consistent with the results of previous studies [4,13].

Cobalt-based catalysts promote the catalytic oxidation of MgSO₃ by initiating a chain reaction. During this reaction, a series of free radicals are generated [18], including sulfite radicals ($\cdot {\text{SO}}_3^{-}$) and peroxysulfate radicals ($\cdot {\text{SO}}_5^{-}$), which could react rapidly to ultimately form ${\text{SO}}_4^{2-}$. This process has been reported in previous related studies [12,18]. Based on this, the oxidation mechanism of Co/AC-Hy catalysts was proposed. The primary reaction pathways are as follows: (i) The reactants O₂ and MgSO₃ are adsorbed onto the catalyst. (ⅱ) The adsorbed MgSO₃ is activated and decomposed into $\cdot {\text{SO}}_3^{-}$ on active cobalt. (ⅲ) The generated chemically reactive $\cdot {\text{SO}}_3^{-}$ species react with O₂ to form $\cdot {\text{SO}}_5^{-}$ and convert into ${\text{SO}}_5^{2-}$. (ⅳ) The ${\text{SO}}_5^{2-}$ species then react with ${\text{SO}}_3^{2-}$ to generate ${\text{SO}}_4^{2-}$ and liberation from the catalyst surface. This reaction process can be represented in Figure 5(c) and was confirmed by previous studies [8,12]. The Co-activated sulfite process can be explained through the non-radical-radical binding theory. This process typically encompasses three main reaction steps: (i) Co²⁺ complexation with ${\text{SO}}_3^{2-}$; (ii) According to Marcus theory, the conformational rearrangement drives single-electron transfer within the Co-S(Ⅳ) complex; (iii) Generation of $\cdot {\text{SO}}_3^{-}$ and $\cdot {\text{SO}}_5^{-}$ by a free-radical chain reaction. Previous research has supported this process [50]. The Co²⁺ generates electron holes $\cdot {\text{SO}}_3^{-}$, which then produce $\cdot {\text{SO}}_3^{-}$ to initiate the free radical chain reaction. As a result, the catalyst easily activates the process, promoting the formation of a sulfite intermediate and accelerating the oxidation of MgSO₃.